1. Field of the Invention
This invention relates to methods of forming plastic composites comprising a filler and a thermoplastic polymer. More specifically, the present invention relates to methods of dispersing and exfoliating fillers in thermoplastic polymers.
2. Background Art
Nanocomposites are a class of materials that can address many of the challenges currently presented by automotive plastics and composites needs. These materials offer a variety of desirable properties including: low coefficient of thermal expansion, high heat deflection temperatures, lightweight, improved scratch resistance, and potential application in automotive Class A surfaces. Nanocomposites are polymers reinforced with nanometer sized particles, i.e., particles with a dimension on the order of 1 to several hundred nanometers. These materials can be used in structural, semi-structural, high heat underhood, and Class A automotive components, among others. Polyolefin based nanocomposites, in particular, have long been sought after due to polyolefin""s wide usage and low resin cost. The major difficulty lies in generating a well-dispersed, well-exfoliated sample due to differences in polarity and compatibility between the clay and polymer phases.
Reinforced plastic materials are continually finding new uses in automotive components. These materials have certain advantages over metals which include higher impact loads before deformation, lighter weight, increased design flexibility, and corrosion resistance. Automotive structural applications have traditionally been made from continuous glass mat composites and highly filled plastic materials such as sheet molding compound (xe2x80x9cSMCxe2x80x9d) where the polymeric component can be as little as 15% by weight. Both SMC and glass mat composite materials (xe2x80x9cGMTxe2x80x9d), however, are still relatively high in density.
Automobile trim and semi-structural components, on the other hand, are commonly fabricated from injection moldable thermoplastics and thermosets. These lighter weight composites, such as short fiber and mineral filled thermoplastics, could be substituted for metals or SMC and GMT composites in the same applications if their mechanical properties could meet the more stringent requirements. Virtually all bumper fascias and air intake manifolds have transitioned from metallic materials to plastics. As new plastic-based materials are developed, the transition will also encompass both more structural components, as well as Class A body panels and high heat underhood applications.
Injection moldable thermoplastics have long been mechanically reinforced by the addition of particulate and fiber fillers in order to improve mechanical properties such as stiffness, dimensional stability, and temperature resistance. Typical fillers include chopped glass fiber and talc, which are added at filler loadings of 20-40% in order to obtain significant mechanical reinforcement. At these loading levels, however, low temperature impact performance and material toughness are sacrificed. Polymer-silicate nanocomposite materials can address these issues.
Polymer-layered silicate nanocomposites incorporate a clay filler in a polymer matrix. Two groups of clay are currently recognizedxe2x80x94the kaolin group and the montmorillonite group. The molecules of the kaolin are arranged in two sheets or plates, one of silica and one of alumina. Similarly, montmorillonite clays are arranged in two silica sheets and one alumina sheet. The molecules of the montmorillonite clays are less firmly linked together than those of the kaolin group and are thus further apart. Composites incorporating either of these clays are potentialcandidates for structural, semi-structural, and Class A vertical and horizontal body applications. Nanocomposites have enjoyed increased interest since the initial development of nylon based material by Usuki et al in 1993. (Usuki, A., et al., Journal of Materials Research, 1993.8 (5): p. 1179-1184.) Typically, polymer nanocomposites combine an organic polymer with an inorganic layered silicate (in the work of Usuki et al., the thermoplastic material Nylon 6 and a montmorillonite clay). Layered silicates are made up of several hundred thin platelet layers stacked into an orderly packet known as a tactoid. Each of these platelets is characterized by large aspect ratio (diameter/thickness on the order of 100-1000). Accordingly, when the clay is dispersed homogeneously and exfoliated as individual platelets throughout the polymer matrix, dramatic increases in strength, flexural and Young""s modulus, and heat distortion temperature are observed at very low filler loadings ( less than 10% by weight) due to the large surface area contact between polymer and filler. The Nylon 6 nanocomposites generated by Usuki were produced by intercalation of caprolactam monomers into the silicate galleries and then in situ polymerization of the monomers. While melt compounding of Nylons with organically modified clays (nanoclays) has also been attempted, the mechanical properties and degree of clay dispersion and exfoliation are slightly short of those of the in situ polymerized type. Efforts to generate similar nanocomposites using other types of thermoplastics and thermosets have enjoyed varying degrees of success.
Due to the polar nature of layered silicates, attempts to generate nanocomposites in a non-polar polyolefin matrix have been only marginally successful. Many research groups have attempted melt compounding of polypropylene and polyethylene based nanocomposites by adding maleic anhydride grafted polypropylene oligomers (PP-MA) to aid in compatibilization and dispersion. While this strategy is somewhat effective in improving nanoclay exfoliation, it requires almost 25% PP-MA, which has the deleterious effect of softening the matrix. To circumvent this issue, a few groups have attempted intercalation of olefin monomers and in situ polymerization to generate polyolefin-silicate nanocomposites. In 1996, Tudor attempted in situ polypropylene polymerization with a Ziegler-Natta catalyst, which produced oligomers, but did not succeed in producing an intercalated or exfoliated structure due to catalyst instability. (Tudor et al., J., et al., Chemical Communications, 1996. v. 17, p. 2031-32.) In 1999, Bergman was able to generate an exfoliated polyethylene by in situ polymerization with a new class of catalyst. (Bergman, J. S., et al., Chemical Communications, 1999.21: p. 2179-2180.) Polypropylene nanocomposites, however, have yet to be generated by in situ polymerization.
For the reasons set forth above, there exists a need for an improved process for dispersing and exfoliating filler material in a polymer matrix.
The present invention overcomes the problems encountered in the prior art by providing a method of dispersing and exfoliating a filler in a polymer matrix by sonicating a mixture of the filler and polymer. The method of the present invention comprises:
a) sonicating a polymer mixture comprising a thermoplastic polymer and a filler at a sonic energy level sufficient to disperse the filler within the thermoplastic polymer;
wherein the thermoplastic polymer is in a melted state during the sonication. This method is particularly useful for dispersing and exfoliating a layered silicate in a polymer matrix.
In another embodiment of the present invention, a polymer-filler composite is provided. The polymer-filler composite of the present invention is characterized as having a filler dispersed within a thermoplastic polymer by the method set forth above. The polymer-filler composite of the present invention is preferably used to form a molded part by method such as injection molding, compression molding, blow molding, and the like.
In yet another embodiment of the present invention a sonic mixing apparatus is provided. The sonic mixing apparatus is preferably combined with a number of plastic mixing or molding equipment. Examples of such equipment include, but are not limited to extruders, injection molding equipment, compression molding equipment, blow molding equipment, and the like.